Nicotinamide Mononucleotide Adenylyltransferase (NMNAT) Maintains Active Zone Structure by Stabilizing Bruchpilot. Shaoyun Zang¹, Yousuf O. Ali², Ruan Kai¹, R Grace Zhai¹. 1) University of Miami,1600 NW 10 Ave. R.M.S.B. Bldg. 6068, Miami, FL 33136; 2) Baylor College of Medicine, Jan and Dan Duncan Neurology Institute, 1250 Moursund, Houston TX 77025.

   Active zones are highly specialized presynaptic sites for synaptic vesicle docking and fusion. Such efficient and precise neurotransmission relies on the structural integrity of active zones. However, the mechanism for maintaining the structural integrity of active zones is largely unknown. Chaperones have been implicated in synaptic function and it is likely that molecular chaperones, the primary machinery that maintains cellular protein homeostasis, play a role in facilitating the redistribution of synaptic proteins and maintaining synaptic structural integrity during neuronal activity. We examined the role of a newly identified chaperone NMNAT (nicotinamide mononucleotide adenylyltransferase) in active zone maintenance. Our previous work has shown that NMNAT is a neuroprotective factor required for maintaining neuronal integrity, including active zone integrity and the neuroprotective ability of NMNAT was attributed partly to its chaperone function. Enzyme-inactive NMNAT rescues active zone degeneration in nmnat null background, suggesting that the chaperone function of NMNAT is sufficient to maintain active zone structure integrity. We directly examined the specific role of NMNAT at the synapse, and identified a novel mechanism of active zone maintenance by NMNAT in which it stabilizes the primary active zone structure protein Bruchpilot (BRP). Loss of NMNAT induced a significant reduction in synaptic BRP levels, leading to accumulation of ubiquitinated BRP, clustering with stress-induced Hsp70 chaperone and a surprising redistribution of BRP from the synapse to the cell body, resulting in the subsequent degeneration of active zones. Moreover, we show that NMNAT interacts with BRP biochemically in an activity-dependent manner. Our findings suggest that NMNAT functions to stabilize BRP and shield it from activity-induced ubiquitin-proteasome-mediated protein degradation, thereby maintaining active zone structural integrity during neuronal activity.